CsvToContacts.cls

// CSV data for Contacts
String inputCsv = 'first_name,last_name,email\nCodey,"The Bear",codey@salesforce.com'; 

DataWeave.Script script = new DataWeaveScriptResource.csvToContacts();
DataWeave.Result dwresult = script.execute(new Map<String, Object>{'records' => inputCsv});
List<Contact> results = (List<Contact>) dwresult.getValue();

Assert.areEqual(1, results.size());
Contact codeyContact = results[0];
Assert.areEqual('Codey', codeyContact.FirstName);
Assert.areEqual('The Bear', codeyContact.LastName);
Assert.areEqual('codey@salesforce.com', codeyContact.Email);

csvToContacts.dwl

%dw 2.0
input records application/csv
output application/apex
---
records map(record) -> {
 FirstName: record.first_name,
 LastName: record.last_name,
 Email: record.email
} as Object {class: "Contact"}